Innate - short of Eldar Jetbikes this fella is about as fast as they come.

Tactical Flexibility

The Piranha will almost never be the focus of your army, however they come with plenty of flexibility. Without upgrades, they are pretty much an anti-infantry harassment unit, with a burst cannon and a pair of twin-linked gun drones meaning that the model is capable of kicking out a respectable 8 S5 shots, with a mix of BS3 and BS2 twin-linked meaning you should see about half of those hit. For a modest cost, you can switch out the burst cannon for a fusion blaster, which if you’re taking the hunter cadre is well worth doing, as it gives that gun a double-dice penetration range of 39” (providing you start close enough to your commander), which will cover most of the board. The piranha can also bring a couple of seeker missiles along for the ride, which, quite frankly, make them a sickeningly effective threat given their modest cost. Thanks to the miracles of markerlights these can target anything on the board just about, hitting on 2+ and ignoring cover at S8 and AP3. In my view, they make the perfect launch platform as the piranha is both small enough to hide fairly easily, and fast enough to be able to influence the game later even if you do start well away from danger. My personal favoured target from them is the ever-annoying thunderfire cannon - a couple of piranhas loosing seeker missiles via markerlights should easily be able to take one down in a single turn, and that’s more than doubling your return on investment.

The Piranha quite simply has all the necessary tools to take on just about any target in the game, though you probably want to steer clear of gargantuan creatures, unless you’re bringing a big squad with lots of seekers!

Cost

40pts basic, up to a frankly ridiculous 125 if you take everything it can have.

Actual cost

Realistically for an open-topped, 2HP skimmer you don’t want to spend too much on them. The basic model is fine if you want to target infantry, 50pts for a tank hunter is quite reasonable and 66pts gets you twin seeker missiles as well. A disruption pod for a 3+ jink would take you up to 81, which is probably getting a bit much for the chassis really, so I’d say go either/or with the disruption pod and the seeker missiles.

Synergy

Markerlights. Say no more! Ok, maybe I can say more here - because the drones can be dropped off to allow them to target a different unit, there’s no need to waste the Piranhas firepower, and it’s cheap enough to be used as somewhat of a sacrificial unit if you’re looking to pop transport tanks. One thing the Tau do kind of lack is a longer ranged early turns anti-tank (genuinely here, not just high strength) weapon, and I think that the Piranha used in combination with the hunter cadre rules grant them that option.

Essential, desirable or whimsical?

Do you need it? Absolutely not. However, if you’re looking to build a hunter cadre on the cheap then the Piranha is your cheapest option (granted, not by much, but every point counts in a Tau list I find). I think given the roles I’ve outlined above though, the Piranha is very much a desirable unit, and can really help the Tau in Maelstrom missions, where a lot of their firepower can be quite static otherwise.

How many dice will you need?

If you maxed out the unit’s potential, then it’s perfectly feasible to get up to 40 shots in a single phase, although you’ll be rolling them in two lots of 20 thanks to the way shooting works now (unless you have 2 sets of 20 dice to differentiate pulse carbine fire, which causes pinning, from burst cannon fire, which doesn’t)

Failure likelihood

Pretty hit and miss here, the anti-infantry version does bring a lot of shots so can be fairly reliable, however if you swap out the burst cannon for a fusion blaster you’re relying on a 1-shot BS3 weapon. I’m just sayin!

Cheese level

Depending on the loadout, a fairly mild cheddar for the basic anti-infantry kit, to a pretty strong cheddar if you’re looking to hide out and use markerlights to launch krak-missile equivalents around the board before zipping out to bust tanks and claim objectives.